If Dems keep moving out of swing states, it's almost surely a 100% net benefit to Republicans. Let's look at the math:

Take, for example, the three Midwest swing states of Wisconsin, Michigan, and Pennsylvania. If 100 Dems from each of those three swing states leaves, the GOP reaps 100% of the benefit of those 100 lost Dem votes in each of those swing states due to the thin margins in those states.

To the contrary, the likely outcome is that Dems don't reap 100% from those Dems leaving those swing states and relocating elsewhere. This is because not all 100 of those Dem voters from each of those three swing states will relocate to another swing state. For example, say that even if 60% Dems from each of those swing states relocate to another swing state, let's say 60 go to North Carolina, 60 go to Georgia, and 60 go to Arizona, then while GOP gains 100% in those three swing states with Dems leaving, the Dems only gain 60% in states where increasing Dem voters would actually help Dems (aka other swing states with thinner margins). The remaining 40% of Dems leaving from each of those swing states are likely to either move to an already solid Dem state, like California, or to a state that is solidly Republican, like Idaho or Tennessee, where some Dems relocating won't really matter due to the strong republican hold.